1. Field of the Invention:
The invention relates generally to the field of infrared imaging systems for displaying a visible image of the infrared or thermal radiation image of a real scene, and, more particularly, to such a system in which the scene is simultaneously illuminated by a coded light beam, thus making possible the simultaneous acquisition of a reflected light image of the scene whose thermal energy is being sensed.
2. Description of the Prior Art:
There are three general approaches to target acquisition with the aid of electro-optical devices, with a first of these involving the viewing of a scene under ambient illumination, and then converting the acquired information into an electronic signal by the use of a photocathode. TV and low light level TV (LLLTV) are representative of this type of approach.
Another approach involves viewing a scene in which artificial illumination, such as supplied by a GaAs diode, is utilized to augment or entirely replace ambient illumination. Laser-aided, range-gated TV's are representative of this second approach.
The third approach involves viewing the scene by means of its natural thermal radiation, and then converting the information by the use of infrared-sensitive detectors into an electronic signal. Forward looking infrared (FLIR) devices are commonly used in accordance with this technique.
In all three approaches, the electronic signals are suitably processed, intensified, and then reconverted into a brightness pattern which is presented as a display to the operator. Target acquisition decisions involving detection, aspect and pattern recognition, and identification are made by the operator.
A serious deficiency associated with the first approach described above involves the fact that ambient illumination goes through extreme dynamic range variations in the course of the daily and monthly natural light level cycles. Furthermore, the daily and monthly light level cycles are accompanied by spectral distribution cycles, so no one photocathode selection, intensifier technique, or optical design is adequate to cope with these extremes.
The second approach described above operates optimally under natural ambient illumination conditions corresponding to a quarter moon or less, but commonality of day and reduced light level optical systems design remains a problem. The required range-gating creates search and detection problems that can be considerable.
The third approach involving the use of FLIR'S has proven itself operationally to have outstanding day/night detection and aspect recognition performance. However, FLIR'S often suffer from deficiencies in pattern recognition, and quite importantly, they do not have "identification" capability, particularly against terrestrial targets. Also, in the course of a day, there are two periods of thermal washout where detection can become problematical, particularly for stationary targets. Washout is also a factor in rainy weather, for rain tends to remove temperature differences.
It is known in the prior art to acquire a remove target by passively sensing it by means of its thermal radiation, and then after acquisition, using a laser to determine the range of the target. Such a system is disclosed in U.S. Pat. No. 3,644,043.
Acquisition of a remote target or scene by simultaneous acquisition of both the radiated thermal and reflected laser energy images has not been possible in the prior art with low power lasers inasmuch as the radiated thermal energy of a scene, especially in terrestrial scenes, is very intense and tends to obliterate the reflected laser energy image, unless the illuminating laser beam is made sufficiently intense so as to be seen as a signal superimposed on the radiated thermal signal. Furthermore, the radiated thermal and laser illuminating signals often have opposite polarities or senses of brightness so as to tend to cancel each other out in an energy detector employed to detect both the radiated thermal energy and the reflected laser energy.
It is most important to note in the utilization of FLIR systems that the problems encountered are often not inherent in the FLIR, but rather involve the nature of the thermal signatures of the scene. The problems of pattern recognition and thermal washout could be resolved if the FLIR could alternatively view, not only the thermal radiation from the scene, but also reflected radiation from an artificial light source, such as a CO.sub.2 laser co-located with the FLIR. By viewing the scene inflectance, the FLIR could then discern features which have no significant thermal signature and which, therefore, lead to faulty pattern recognition and thermal washout. Furthermore, target acquisition time line improvements will be observed when passive and active viewing capabilities are utilized cooperatively.
In order to implement such a dual capability FLIR, it is important to observe several criteria. First of all, the CO.sub.2 laser illumination must scan synchronously with the FLIR detector array. Secondly, the FLIR should simultaneously sample the thermal and CO.sub.2 laser signals. Thirdly, the FLIR must utilize signal processing which allows it to discriminate between thermal and CO.sub.2 laser signals. It is important to note that, if these latter three criteria are not met simultaneously, then excessive CO.sub.2 laser power is required, the FLIR performance is compromised, and the thermal and CO.sub.2 laser signals may tend to conflict with each other, thus leading to another kind of washout.